Fix OST pool grow

[ip1981]

Different types of sets were compared to each other:
objects and strings:

current set([<ManagedOst: fs-OST0000>]),
new set([u'fs-OST0000', u'fs-OST0001']),
add set([u'fs-OST0000', u'fs-OST0001']),
remove set([<ManagedOst: fs-OST0000>]).

Make sure to compare strings only.

There is a minor and likely unrelated issue when growing with the same OST: an empty job list results in an error:

[root@adm ~]# vim /var/log/chroma/job_scheduler.log
[root@adm ~]# iml filesystem pool grow fs fast fs-OST0002
Growing ost pool...
[1/1]   ✔ Updating OST Pool successful
[root@adm ~]# iml filesystem pool grow fs fast fs-OST0001
Growing ost pool...
✗ HTTP status client error (404 Not Found) for url (https://adm.local/api/ostpool/4/)
[root@adm ~]# iml filesystem pool show fs fast
+------------+------------+
| Filesystem | fs         |
+------------+------------+
| Name       | fast       |
+------------+------------+
| OSTs       | fs-OST0000 |
|            | fs-OST0001 |
|            | fs-OST0002 |
+------------+------------+
[root@adm ~]# 

Closes #1852 and probably #1853:

chroma=> select * from chroma_core_ostpool;
 id | name | not_deleted | content_type_id | filesystem_id 
----+------+-------------+-----------------+---------------
  1 | fast |             |              44 |             1
  2 | fast |             |              44 |             1
  3 | fast |             |              44 |             1
  4 | fast | t           |              44 |             1
(4 filas)

chroma=> select * from chroma_core_ostpool_osts;
 id | ostpool_id | managedost_id 
----+------------+---------------
 17 |          4 |            10
 18 |          4 |            12
 19 |          4 |            13

Signed-off-by: Igor Pashev pashev.igor@gmail.com

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This change is 

[jgrund]

Change looks good, waiting on full test run pass